The ground-state properties of the half-filled Hubbard model are investigated in the framework of lattice density functional theory. The single-particle density matrix ${\gamma }_{ij\sigma }$ is regarded as the central variable of the many-body problem, where $i$ and $j$ refer to the lattice sites and $\sigma$ to the spin. The interaction-energy functional $W\left[{\gamma }_{ij\sigma }\right]$ is calculated exactly for representative finite periodic systems by performing exact Lanczos diagonalizations. The relationship between $W\left[{\gamma }_{ij\sigma }\right]$ and the entropy $S\left[{\eta }_{\mathit{k}\sigma }\right]$ of independent fermions with natural-orbital occupations ${\eta }_{\mathit{k}\sigma }$ is analyzed. A simple approximation to the interaction energy of the half-filled Hubbard model is proposed, which takes the form $W=W\left(S\left[{\eta }_{\mathit{k}\sigma }\right]\right)$. Using this functional we derive the ground-state energy, kinetic energy, average number of double occupations, charge distribution, magnetic susceptibility, and field-induced spin polarization in one-, two-, and three-dimensional periodic lattices. The limit of infinite dimensions is also explored. The accuracy of the method is assessed by comparison with available exact numerical or analytical results. Goals, limitations, and possible extensions of the domain of applicability of the functional are discussed.

The combined effect due to mechanical strain, coupling to the plasmons in a doped conducting substrate, and the plasmon-phonon coupling in conjunction with the role played by encapsulation of a secondary two-dimensional layer is investigated both theoretically and numerically. The calculations are based on the random-phase approximation for the surface response function which yields the plasmon dispersion equation that is applicable in the presence or absence of an applied uniaxial strain. We present results showing the dependence of the frequency of the charge density oscillations on the strain modulus and direction of the wave vector in the Brillouin zone. The shielding of a dilute distribution of charges as well as the rate of loss of energy for impinging charges is investigated for this hybrid layered structure.

We present a study of (110)-terminated magnetite using a range of surface-sensitive techniques, the foremost of those being reflectance anisotropy spectroscopy (RAS). The anisotropic optical response of the as-polished and row-reconstructed terminations are investigated. The responses are interpreted to result from termination-induced anisotropic shifts in energy of the bulklike optical transitions. Density functional theory (DFT) calculations demonstrate that the row reconstruction of ${\mathrm{Fe}}_3{\mathrm{O}}_4$(110) exhibits anisotropic strain in its terminating layers, and this strain is concluded to be the origin of the RAS response. X-ray photoelectron spectroscopy measurements demonstrate that the row reconstruction is reduced and the stoichiometry in the surface region is relatively easily altered by in situ preparation procedures. This is correlated to altered surface electronic properties identified by scanning tunneling spectroscopy measurements. RAS is sensitive to this change in surface structure, with the magnitude of the RAS response being altered.

The recently discovered $(\mathrm{Rb},\mathrm{Cs}){\mathrm{EuFe}}_4{\mathrm{As}}_4$ compounds exhibit an unusual combination of superconductivity ($T_c\sim 35\mathrm{K}$) and ferromagnetism ($T_m\sim 15\mathrm{K}$). We have performed a series of x-ray diffraction, ac magnetic susceptibility, dc magnetization, and electrical resistivity measurements on both ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ and ${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$ to pressures as high as $\sim 30\mathrm{GPa}$. We find that the superconductivity onset is suppressed monotonically by pressure while the magnetic transition is enhanced at initial rates of $d{T}_m/dP\sim 1.7\mathrm{K}/\mathrm{GPa}$ and $1.5\mathrm{K}/\mathrm{GPa}$ for ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ and ${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$, respectively. Near 7 GPa, $T_c$ onset and $T_m$ become comparable. At higher pressures, signatures of bulk superconductivity gradually disappear. Room-temperature x-ray diffraction measurements suggest the onset of a transition from tetragonal (T) to a half-collapsed-tetragonal (hcT) phase at $\sim 10\mathrm{GPa}$ (${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$) and $\sim 12\mathrm{GPa}$ (${\mathrm{CsEuFe}}_4{\mathrm{As}}_4$). The ability to tune $T_c$ and $T_m$ into coincidence with relatively modest pressures highlights $(\mathrm{Rb},\mathrm{Cs}){\mathrm{EuFe}}_4{\mathrm{As}}_4$ compounds as ideal systems to study the interplay of superconductivity and ferromagnetism.

Under the assumption of two-fluid kinematics of a nonrelativistic Bose liquid in the presence of a local velocity field $v\left(x\right)$, local Galilei transformations are used to derive formulas for the spatial distribution of superfluidity. The local formulation is shown to subsume several descriptions of superfluidity, from Landau's free quasiparticle picture of the normal fluid to the fully microscopic winding number formula for superfluid density. We derive the superfluid distribution of generic pure states of one-dimensional bosonic systems by using the continuum analog of matrix product states. With a view toward spatially structured superfluid-based quantum devices, we consider the limits to local distillation of superfluidity within the framework of localized resource theories of quantum coherence.

Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an $s+is$ superconducting state that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal symmetry breakdown. We find that two-band superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second-order phase transition from $s_{\pm }/s_{++}$ states to the $s+is$ state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.

In the Kerr rotation geometry, magneto-optic memory devices typically suffer from low figure-of-merit (FOM) and long write times. We show that skyrmions formed at the interface of a thin-film multiferroic and a topological insulator can give rise to high FOM magneto-optic Kerr effects (MOKEs). Huge differential MOKE can arise in parts of the phase diagram. Resonancelike features in the MOKE spectra arising from the induced low energy TI band gap, the multiferroic-film thickness, and the high energy Drude-like behavior are resolved and explained. The Fermi level dependence of the MOKE signatures is distinct for the different magnetic textures. This has broad implications for magnetic texture characterization, electro-optic modulators and isolators, and high density magnetic optic memory.

The origin of the unusual second magnetic transition at a temperature $T_M$ in the ruthenate $\mathrm{S}{\mathrm{r}}_4\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{10}$ remains elusive. Here, we have investigated the thickness-dependent anisotropic magnetoresistance of $\mathrm{S}{\mathrm{r}}_4\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{10}$ nanosheets, and clearly found that the sign of the magnetoresistance (MR) presents an unusual reversal behavior from a positive to negative with an external magnetic field $H\perp ab$ plane or vice versa with $H\parallel ab$ plane when the temperature decreases across the $T_M$. Analysis of the data demonstrates that this MR reversal is caused by the magnetization reversal from the $ab$ plane ferromagnetic order at $T>T_M$ to the $c$ direction at $T<T_M$ due to the competition between the shape anisotropy and the inherent magnetocrystalline anisotropy. Our result suggests that the second magnetic transition in $\mathrm{S}{\mathrm{r}}_4\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{10}$ nanosheets originates from a spin reorientation.

We study spin-polarized quasiparticle transport in a mesoscopic superconductor with a spin-splitting field in the presence of coflowing supercurrent. In such a system, the nonequilibrium state is characterized by charge, spin, energy, and spin-energy modes. Here we show that in the presence of both spin splitting and supercurrent, all these modes are mutually coupled. As a result, the supercurrent can convert charge imbalance, which in the presence of spin splitting decays on a relatively short scale, to a long-range spin accumulation decaying only via inelastic scattering. This effect enables coherent charge-spin conversion controllable by a magnetic flux, and it can be detected by studying different symmetry components of the nonlocal conductance signal.

Protected zero modes in quantum physics traditionally arise in the context of ground states of many-body Hamiltonians. Here we study the case where zero modes exist in the center of a reflection-symmetric many-body spectrum, giving rise to the notion of a protected “infinite-temperature” degeneracy. For a certain class of nonintegrable spin chains, we show that the number of zero modes is determined by a chiral index that grows exponentially with system size. We propose a dynamical protocol, feasible in ongoing experiments in Rydberg atom quantum simulators, to detect these many-body zero modes and their protecting spectral reflection symmetry. Finally, we consider whether the zero-energy states obey the eigenstate thermalization hypothesis, as is expected of states in the middle of the many-body spectrum. We find intriguing differences in their eigenstate properties relative to those of nearby nonzero-energy eigenstates at finite system sizes.

It is well known that surface melting of metal materials is caused by vacancies, and melting proceeds layer by layer in theoretical predictions. However, the melting process has rarely been directly investigated in real time at atomic resolution. Herein, the (200) surface-melting process of Cu nanoparticles with sizes of about 50 nm at ${750}^{\circ }\mathrm{C}$ was first observed by in situ heating transmission electron microscopy. Initially, surface-melting nucleation occurs at the edge and corner of one side of the Cu(200) surface. Subsequently, the nucleated region size increases to a critical value (about 14 layers). Finally, collapse-type melting rapidly extends to the whole Cu(200) surface. This surface-melting process repeatedly occurs. This work will enhance the understanding of the surface-melting mechanism and provide a theoretical foundation to avoid the collapse of Cu nanomaterials during high-temperature applications.

Using a multiple scattering technique, we derived closed-form expressions for effective constitutive parameters and electro/magneto-strictive tensor components for 2D bianisotropic metamaterials. Using the principle of virtual work, we obtained the electromagnetic stress tensor that can be used to calculate the optical force density inside such media. The analytic expressions are tested against full wave numerical simulations. Our effective medium theory is essential for providing a complete macroscopic description of the optical and opto-mechanical properties of bianisotropic composites.

Heterostructures of topological insulators and ferromagnets offer new opportunities in spintronics and a route to novel anomalous Hall states. In one such structure, EuS/${\mathrm{Bi}}_2{\mathrm{Se}}_3$, a dramatic enhancement of the Curie temperature was recently observed. We performed Raman spectroscopy on a similar set of thin films to investigate the magnetic and lattice excitations. Interfacial strain was monitored through its effects on the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ phonon modes while the magnetic system was probed through the EuS Raman mode. Despite its appearance in bare EuS, the heterostructures lack the corresponding EuS Raman signal. Through numerical calculations we rule out the possibility of Fabry-Perot interference suppressing the mode. Direct measurements of the magnetic system also eliminate room temperature ordering from suppressing the mode. We therefore attribute the absence of a magnetic signal in EuS to a charge transfer with the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. This could provide an additional pathway for manipulating the magnetic, optical, or electronic response of topological heterostructures.

We study theoretically the light-induced magnetization switching in a binary ferrimagnet of the type ${\mathrm{A}}_p{\mathrm{B}}_{1-p}$, randomly occupied by two different species of magnetic ions. The localized spins are coupled with spins of itinerant electrons via the $s\text{−}d$ exchange interaction. The dynamics of the localized and itinerant spins is described by coupled rate equations, which include electron-phonon interaction, spin-lattice relaxation, and exchange scattering, induced by the $s\text{−}d$ exchange interaction. The exchange scattering leads to the formation of ferromagneticlike states at initial temperatures $T$ both below and above the magnetization compensation temperature $T_M$ with the opposite polarities in these two temperature regions. Inclusion of electron-phonon interaction and spin-lattice relaxation in the dynamical equations leads to the switching in a temperature range $0<T<T_f$, where $T_f$ is slightly higher than $T_M$ and strongly depends on the spin-lattice relaxation time of itinerant electrons. The switching requires less pulse fluence in the vicinity of $T_M$.

Layered $1T\text{−}{\mathrm{TiSe}}_2$ has attracted much interest for the competition of charge density wave (CDW) and superconductivity in its bulk and even monolayer forms. Here we perform first-principles calculations of the electronic structure, phonon dispersion, and electron-phonon coupling of the Pb-intercalated $1T\text{−}{\mathrm{TiSe}}_2$ in bulk and layered structures. Results show that upon the Pb atom intercalation, the CDW instability in $1T\text{−}{\mathrm{TiSe}}_2$ can be effectively suppressed, accompanied by the removal of the imaginary phonon modes at q${}_{\mathrm{M}}$. The Pb $6p$ orbitals occupy directly at the Fermi level. Both bulk and layered ${\mathrm{PbTiSe}}_2$ are phonon-mediated superconductors, with estimated superconducting temperature $T_c$ to be $\sim 1.6$–3.8 K. The main contribution to the electron-phonon coupling is from the vibrations of Pb and Se atoms and the superconductivity is mainly raised by Pb. The superconducting related physical quantities are found tunable by varying Pb content.

We propose a model and derive analytical expressions for conductivity in heterogeneous fully anisotropic conductors with ellipsoid superconducting inclusions. This model and calculations are useful to analyze the observed temperature dependence of conductivity anisotropy in various anisotropic superconductors, where superconductivity onset happens inhomogeneously in the form of isolated superconducting islands. The results are applied to explain the experimental data on resistivity above the transition temperature $T_c$ in the high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ and in the organic superconductor $\beta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$. The comparison of resistivity data and diamagnetic response in $\beta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ allows us to estimate the size of superconducting inclusions as $d\sim 1\mu \text{m}$.

Nematicity is ubiquitous in electronic phases of high-$T_{\mathrm{c}}$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum-averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.94}{\mathrm{Co}}_{0.06}\right)}_2{\mathrm{As}}_2$. We determine the nematic correlation length and find that it sharply changes its $T$ dependence at $T_{\mathrm{c}}$, revealing a strong connection between nematicity and superconductivity.

We present a detailed investigation of the electronic properties of $\mathrm{CeFeAsO}$ under in-plane chemical (As by P substitution) and hydrostatic pressure by means of in-house and synchrotron Mössbauer spectroscopy. The Fe magnetism is suppressed due to both pressures and no magnetic order was observed above a P-substitution level of 40% or 5.2 GPa hydrostatic pressure. We compared both pressures and found that the isovalent As by P substitution changes the crystallographic and electronic properties differently than hydrostatic pressure.